Ulrasonic technology has in recent years become ever more important in medical diagnosis. Such technology finds application where it is desired to examine internal body organs and fluids with the objective of locating features or aspects which may be indicative of disease or absnormalities. Typical instruments detect the amplitude of the echo signals returning from the structure being examined, and usually display the information in a two-dimensional "B Scan" image. Less common, and less straightforward, is the detection of velocity along the axis of the interrogating sound beam, rather than amplitude. Such detection can provide an image of the blood flow pattern, or vessel network, information of high diagnostic significance. The detection of velocity is based upon the Doppler principle, whereby a change in observed frequency of the reflected echo pulse is indicative of a corresponding change in the velocity in the region from which the echo emanates.
Fortunately, much basic work on such Doppler based flowmeter systems has already been done. A basic system is described in M. Anliker, titled "Current and Future Aspects of Biomedical Engineering", Triangle, Volume 16, No. 3/4, 1977, 129, 130-132. Another later system of the type is described in M. Brandestini, "Topoflow-A Digital Full Range Doppler Velocity Meter", IEEE Transactions in Sonics and Ultrasonics, Volume SU-25, No. 5, September 1978, Pages 288-291. Other similar papers are "Blood Flow Imaging Using a Discrete Time Frequency Meter", Brandestini and Forrester, 1978 Ultrasonic Symposium Proceedings, IEEE Catalog 78CH1344-ISU and F. D. McLeod, M. Anliker, "A Multiple Gate Pulsed Directional Doppler Flowmeter, Proceedings IEEE Ultrasonic Symposium," Miami, December 1971; and F. E. Barber, D. W. Baker, D. E. Strandness Jr. and G. D. Mahler, "Duplex Scanner II", Ultrasonic Symposium Proceeding, IEEE Catalog 74, CHO08961SU, 1974.
Most of the prior Doppler systems involve an RF ultrasonic pulse transmitting and receiving section, and some form of quadrature phase detection, transmitting by means of a transducer an interrogating pulse train into the structure under examination and receiving and resulting echo information for processing. The RF frequency is of the order of megacycles, while the pulse repetition frequencies are typically in the kilohertz range. Thus, a pulse repetition interval of 100 to 200 microseconds between pulses may be expected, and a useful range of no more than 10 to 20 centimeters into the patient's body. Along with the received actual echo signal, the quadrature echo signal, accomplished by mixing with a local oscillator signal differing in phase by 90.degree. from the transmitter frequency, preserves phase and the ability to later detect the sign, i.e., flow direction, of the fluid movement under examination.
The next section typically found in such Doppler systems usually involves the sampling of the Doppler information-carrying envelope with both the original and orthogonal detected echo signal into a number of channels similar to the number of microseconds interval between the pulses, typically into 128 channels. Of course, each such channel or portion of the time interval between pulses also corresponds to a portion or interval of a range within the patient's body under interrogation. If structures or fluids in any such interval within the body have a velocity component in the direction of the axis of ultrasonic radiation, a Doppler frequency change is impressed upon the echo emanting from such interval or channel.
However, complicating the detection of such Doppler frequencies is the existence of quasi-specular stationary reflecting tissue interfaces which yield echo signals of large amplitudes, thus masking the much lower amplitude signals scattered by the moving blood cells, and which actually contain Doppler information of interest. The difference in amplitude may be as much as two orders of magnitude. Such large echo "clutter" signals, being from stationary interfaces, have no Doppler information, and change little, if at all, from one interrogating pulse to the next, while the echo signals from moving scatterers such as blood will change rapidly. It was realized that decomposing the received echo signals into the large, but relatively fixed, clutter components, and a small, rapidly-changing signal component could provide the key to resolving this masking problem.
Accordingly, as the next stage of some Doppler systems, certain investigators, (especially Anliker and Brandestini), utilized analog/digital recursive stationary canceler-filters based on principles first utilized in radar in order to digitize the incoming signals and remove therefrom those components from fixed tissue interfaces, and low-pass filter the non-blood flow low-frequency Doppler signals from such reflectors as moving lumen walls. Such filters attempted to split the analog digital version of the sampled echo signals into a "fast" and a "slow" section, and utilized a tracking type conversion relying on the Doppler difference between subsequent pulses to track and eliminate clutter, since it is nearly constant from pulse to pulse, while attempting to digitize with maximum speed the small-amplitude, but fast-changing Doppler portion of the system remaining after subtraction of the clutter.
Such digital canceller-filters, while a substantial improvement over prior expedients, nevertheless have not performed well enough to enable Doppler flowmeter systems to function at practical levels, with performance levels sufficient to provide truly acceptable commercial instruments. Rather severe demands on such filters result from the fact that not only are the amplitude changes in the echoes as much as two orders of magnitude different, but also such changes are very sudden, and may cause transfer of low frequency energy content into neighboring channels. The handling of such large-amplitude changes obviously requires a high degree of resolution and dynamic range, and the suddenness of such changes further requires short response times, if substantial amounts of Doppler information-bearing echo signals are not to be lost under the influence of clutter signal amplitudes, and because of the time required for the filter to respond and to eliminate same. In these respects, the stationary filter-cancellers of the prior art have been less than satisfactory, and have been found lacking, especially in the dynamic range and response time necessary to perform at a sufficient resolution level.
The filter-canceller stage has then typically supplied the input for a zero crossing detector and a companion flow velocity sign detector circuit. The function of the crossing detector is to detect the zero crossings which the Doppler signal undergoes over a period of time in one direction for each channel, which then gives the measure of the Doppler frequency. The sign detector is important in determining whether a positive or negative Doppler shift is occurring within each channel. This is critical for the operation of the conventional Doppler frequency to velocity converter with which these systems are finally equipped, and which then yield a velocity for each channel. Such converters accumulate over some predetermined time period counts corresponding to the occurrence of zero crossings for each channel, and must be instructed as to whether the zero crossing is in one sense or the opposite sense, that is, whether the count should be added or subtracted. The sign detector, by comparing quadrature components through which phase information has been preserved, and obtaining the instantaneous direction of the Doppler frequency, supplies such instruction to the velocity converter.
Again, while such zero crossing detectors and sign detector means have been the best expedients heretofore available, they, too, have had serious shortcomings. These have primarily to do with the inherent limitations of sample systems, in particular as imposed by the well-known Nyquist criterion. In other words, in quadrature multiplexed systems, it is well-known that when the detected frequency exeeds one-quarter the repetition rate of the interrogating acoustic pulse, the phase information can no longer be preserved. Thus, while the fact of a zero crossing may be reliably detected in such prior art expedients, its direction will not be reliably detected, under the foregoing conditions. Therefore the operation of any Doppler frequency to velocity converter in the prior art under these circumstances is correspondingly also unreliable and unsatisfactory.
Accordingly, it may be regarded as an object of the present invention to provide a Doppler flowmeter system with improved resolution, dynamic range and response time to enable a practical level of performance in measuring the flow of body fluids.
It is a further object of the invention to provide a Doppler system with an improved digital stationary canceler-filter for a Doppler flowmeter system having improved dynamic range and response time at high resolution level.
It is a still further object of the invention to provide a Doppler system with improved zero crossing and sign detector circuits having an extended Doppler bandwidth for improved handling of sampled echo information to assure satisfying the Nyquist criterion.